Having lower back pain and pain in hips and legs. Primarily left hip/leg. What could be the cause of this?

Wait another week. Many herniated discs like what sounds like what you are having will resolve on its own in a few days to weeks. If you are still in a lot of pain and anti-inflammatories are not helping or if you notice numbness/weakness presenting itself then seek medical care to evaluate this further.
Most likely. Musculoskeletal, such as a sacroiliac joint disorder. The sacral bones are below your lumbar or lower back vertebrae and they connect on each side to the pelvic bones. They are held together by very strong ligaments and not much movement occurs there, except at the hips and lumbar spine area, placing a lot of stress at the si joint. Pregnancy due to hormones can also increase stress there.
It's your disc. Your symptoms are most likely related to an intervertebral disc, in which there may be a tear or even herniation. When the symptoms radiate from the back down through the buttock or into the hip/leg region, it suggests that a nearby nerve root is irritated. Seek a spine md if you have any numbness, tingling, weakness, or bowel/bladder problems, or if pain persists despite taking nsaids for >1wk.